1. Field of the Invention
The present invention is concerned with an improved process for forming a deposition film by means of DC or low frequency AC discharge, and more particularly it relates to a process for forming a deposition film which improves the forming speed extraordinarily.
2. Description of the Prior Art
Amorphous material which comprises at least one of silicon or germanium atom as a matrix and contains at least one of hydrogen or halogen atom (identified as X), such as amorphous hydrogenated silicon (hereinafter represented by "a-Si:H") amorphous halogenated silicon (hereinafter represented by "a-Si:X"), amorphous hydrogenated germanium (hereinafter represented by "a-Ge:H"), amorphous halogenated germanium (hereinafter represented by "a-Ge:X") and the like, have many advantages as shown below.
(1) Since the above mentioned amorphous material has less defects (dangling bonds, voids and the like) than conventional amorphous silicon (hereinafter represented by "a-Si") or amorphous germanium (hereinafter represented by "a-Ge"), it exhibits high sensitivity when used as a photoconductor.
(2) By doping an amorphous material with the elements of Group III of the Periodic Table such as boron (B), and elements of Group V such as phosphorous (P), arsenic (As), and the like, in the same manner as in case of crystalline silicon (hereinafter represented by "C-Si") or crystalline germanium (hereinafter represented by "C-Ge"), the type of conductivity may be controlled in either of p-type, n-type or i-type.
(3) Since a vacuum discharge deposition, such as a glow discharge deposition, may be utilized in forming a film having a large surface, the amorphous material is useful for producing an element for solar cell, as target material for an image-pickup tube, and as photoconductive material for an electrophotographic photosensitive member. On the contrary, a deposition speed in forming films such as films of a-Si:H, a-Si:X, a-Ge:H, a-Ge:X and the like according to the above mentioned process, is generally 0.1 to 40.ANG./sec, which is very slow as compared with that of a photoconductive material, such as Se. This slow speed results in low productivity to hamper a cost efficient operation, and causes a great hindrance when a-Si:H, a-Si:X, a-Ge:H, a-Ge:X and the like are used for the purpose of forming the thick film. Particularly, when these materials, a-Si:H, a-Si:X, or a-Ge:H, a-Ge:X, are used as a photoconductive material and are formed in a layer of photoconductive material of an electric photographic photosensitive member, generally employed in the procedure of electrophotography, a photoconductive layer is required to be 10 .mu.m or more in thickness to produce a good image, and thereby, the slow speed of depositing a film on the substrate becomes a more serious problem in practical application.
For instance, on forming a 20 .mu.m thickness of a photoconductive layer of a-Si:H series, or a-Si:X series material, it takes almost 5.5 hours to form a photoconductive layer under a deposition speed of 10 .ANG./sec. Accordingly it requires a long time to form a deposition layer, and the electrophotographic photosensitive member becomes expensive. Consequently, in the case of the glow discharge process which is believed to have high reproducibility and to give a highly sensitive film in forming a film of a-Si:H, a-Si:X, a-Ge:H or a-Ge:X, various approaches concerning improvements in deposition apparatus and deposition speed related to a discharge power; concentration, pressure and flow rate of the raw material gas; frequency of discharge source and a temperature of substrate, have been studied for increasing the deposition speed. As for "a-Si:H", for example, it is disclosed in the paper "Characterization of Plasma-Deposited Amorphous Si:H" by Knight, that by increasing concentration of the material gas and discharge power, the deposition speed increases from 1 .ANG./sec. to 9 .ANG./sec.
It is also disclosed in the paper "large surface area amorphous Si solar cell" by Yoshiyuki Uchida that a deposition speed is increased to 3 .ANG./sec. from 1 .ANG./sec. by changing the pressure of the raw material gas in the range between 2 and 20 Torr, and that increase in deposition speed takes place by improving the design of an inlet port for introducing the raw material gas, an outlet port of exhaust gas, and a discharge electrode. However, at present, it has never been reported that a practical photosensitive film of a-Si:H, a-Si:X, a-Ge:H, a-Ge:X and the like, having a thickness of several tens .mu.m can be deposited within several hours, that is, a process for forming a film continuously during a specified film forming time at a deposition speed of several tens .ANG./sec. has never been reported.
Thus, development a process of forming film in a thickness of several tens .mu.m while maintaining a high deposition speed, has been approached in various fields of arts. However, no excellent forming process for depositing such a film effective for industrial application has ever been proposed.
On the other hand, in the forming process for films of a-Si:H, a-Si:X or a-Ge:H, a-Ge:X by vacuum deposition there are known generally capacitive type and inductive type processes. Among these, when a film of a large surface area having uniform properties is desired, a capacitive type process is generally preferred, and, particularly, application of a capacitive glow discharge is more effective.
The capacitive type process is further classified into a direct current method and an alternating current method.
When a film is formed on the support by decomposing a raw gaseous material to be used for forming film such as, for example, SiH.sub.4, Si.sub.2 H.sub.6 and the like, and by depositing an amorphous material and which comprises silicon atom as matrix, in the alternating current method, the frequency of electric power for generating glow discharge is usually from several MHz to several tens of MHz.
The reason for using such a high frequency is that, since films of, for example, a-Si:H and a-Si:X possess a volume resistance of when high as about 10.sup.7 -10.sup.15 ohm.cm, as being deposited on a substrate or on a discharge electrode being used as a substrate, a stable discharge may be maintained for many hours by preventing an increase in discharge impedance which takes place due when a similar effect to introducing in series a material having relative small capacitance within a discharging zone.
Thus, the alternating current process is advantageous since a stable continuous discharge can be maintained while it is disadvantageous in that the deposition speed is, in general, so slow that it takes a long time to form a thick film.
On the other hand, it is understood that a direct current discharge is highly desirable where a raw gaseous material is effectively decomposed and only the necessary components, among the resultant compounds deposit on the support at a high speed. However, as disclosed above, impedance increases as the deposition proceeds, and it becomes difficult to maintain a stable discharge for many hours.
These effects may be explained in accordance with the accompanying drawings. In FIG. 1, discharge electrodes 2 and 3 face each other at a distance of about 50 mm and the discharge electrode 2 holds a substrate 4 of aluminum, which may be heated with a heater 7 built in a film deposition apparatus 1. The apparatus 1 is exhausted towards the direction of arrow mark B by a vacuum pump to a pressure of 2.times.10.sup.-6 torr. Then an inlet valve 6 is opened to let a raw gaseous material such as Si.sub.2 H.sub.6, SiH.sub.4 and the like into a film deposition apparatus 1, from a direction mark "A" in a flow rate of 50 SCCM. The vacuum pump is controlled to adjust the pressure in film deposition apparatus 1 to 1 torr, and direct current voltage is applied to the discharge electrodes 2 and 3 from direct current power 5 to cause a discharge for forming film.
In FIG. 2, the relation between time of film deposition and discharge current and deposition rate at a discharge voltage of 600 V is represented graphically. As shown in FIG. 2 it is clear that when a glow discharge is utilized, discharge current and deposition rate are decreased suddenly as the deposition proceeds, and finally the discharge ceases spontaneously and the deposition can not be further maintained. For example, in the case of a-Si:H film, the glow discharge will be able to form film thickness of at most several microns.